Typically, in life prediction and integrity evaluation of components or materials, information on a defect position is very important, which requires an accurate and fast detection technique.
Pulse echo technique, which is one of existing nondestructive defect detection techniques, is a technique for detecting a defect according to an amplitude of energy reflected by the defect existing inside an inspection object. However, it is difficult to measure an accurate size of the defect, since the amplitude of the reflected energy is dependent on a state of a reflection surface.
In contrast, as a defect detection technique using a diffraction phenomenon of a wave, time of flight diffraction method (TOFD) has a 10 times or more faster inspection speed than the pulse echo technique, enables an inspector to easily detect a defect, and does not depend on a direction of the defect, and therefore it gets spotlighted as a stable inspection method.
The TOFD is a technique for transmitting a wave to an inspection object, namely, a medium, using a transmission probe and a reception probe and analyzing a defect position such as a discontinuity inside the inspection object. The wave transmitted from the transmission probe passes through the medium to collide with the defect therein, and generate a diffraction wave. The generated diffraction wave is received by the reception probe, and the defect position is detected by measuring a time when the wave is transmitted from the transmission probe and a time when the diffraction wave is received by the reception probe.
However, for the TOFD, since only an overall movement distance between the transmission wave and the diffraction wave generated thereby is measured, it is difficult to accurately measure a three-dimensional position of the defect existing inside the inspection object, which includes a defect position on the surface of the inspection object, a defect depth from the surface of the inspection object, and the like.